This invention relates to a sealing material to be associated with soft glass.
Known sealing materials to be associated with soft glass include an alloy of iron (Fe) - nickel (Ni) - chromium (Cr) system such as 42 Ni - 6 Cr - Fe alloy, an alloy of iron - nickel system such as 48.about.52 Ni - Fe alloy and an alloy of iron - chromium system such as 18.about.27 Cr - Fe alloy. Of these alloys, the iron - nickel - chromium system alloy has been widely used by virtue of its excellent sealing reliability, etc.
When a sealing material made of the iron - nickel - chromium system alloy and soft glass is sealed or adhered to each other, usually the sealing material is first preoxidized in a wet hydrogen furnace to form an oxide layer on its surface and thereafter it is subjected to the sealing to soft glass. Accordingly, it has been a principal technical problem in the conventional alloy of such system that the adhesion between oxide layer formed by preoxidation treatment and the metal underneath should be improved. To solve such problem, it has been proposed that a small amount of aluminum (Al), silicon (Si), vanadium (V), rare earth metals and/or the like is added to the alloy, thereby improving the property of the oxide layer. Although the adhesion has been thereby improved to a certain extent, there has still been room for improvement to meet the need for attaining a sealing state with higher reliability.
In a body constructed by sealing a sealing material to soft glass, it is preferred that the oxide layer on the surface of a sealing material has a thickness as thin as possible. This is because an oxide layer having a greater thickness is liable to be peeled off due to the inherent fragility of the oxide layer, thereby causing destruction of hermetic sealing (i.e. leak) at the peeled portion, and also it results in a greater glass strain caused by the differences in thermal expansion coefficient between the sealing material and the glass.
It is therefore preferred that the oxide layer is as thin as possible and the layer is usually formed with the thickness of about 0.5 to 5.mu.. However, if it is attempted to make the oxide layer much thinner in order to attain more preferred sealing effect, there will occur a phenomenon in which the oxide layer is over-oxidized at the time of sealing procedure (i.e. an over-oxidation phenomenon in which oxidation proceeds in abnormal state in part to form a too thick oxide layer) or a phenomenon in which blooming of iron oxide from base alloy takes place, whereby the sealing strength is impared drastically.
The present inventor, has noted that these phenomena are caused by nonuniform strength of the oxide layer per se to be formed by preoxidation treatment, rather than poor adhesion between the oxide layer and the metal underneath.